The Clarno Formation is represented throughout central Oregon and includes early to middle Eocene age volcanic rocks, debris-flow conglomerates, lahars and volcaniclastic sedimentary rocks interpreted as having been deposited within an extensional basin or series of basins located near sea level at the time (Rogers and Novitsky-Evans 1977; Rogers and Ragland 1980; Noblett 1981; White and Robinson 1992; Bestland et al. 1999; Dillhoff et al. 2009; McClaughry et al. 2009). The source for the Clarno volcanics was a broad local arc of volcanoes resulting from ‘flat slab’ subduction of oceanic crust beneath western North America, occurring from the Cretaceous through the Eocene (White and Robinson 1992). The ash from these eruptions ultimately became the claystone and siltstone deposits that characterise the Clarno Formation, including the repeated palaeosol units near the top of the formation (Bestland et al. 1999). These Clarno Formation deposits were formed prior to the shift of arc magmatism and the formation of the well-known Cascades arc to the west (Fisher 1967; McBirney 1978; Taylor 1990; Bestland et al. 1999).

The fossils described here come from the Hancock Mammal Quarry (HMQ, Fig. 1), which lies within the uppermost strata of the Clarno Formation in the Clarno area. The well-known and stratigraphically lower, Clarno Nut Beds are part of the conglomerates of Hancock Canyon. Multiple radiometric dates published from the Nut Beds itself (Manchester 1994; Bestland et al. 1999) are all approximately 44 Ma. Overlying those units are a series of red and lavender claystone units with deeply weathered palaeosols, including a stony tuff bed dated to 42.7 ± 0.3 Ma (Bestland et al. 1999). That date provides an approximate maximum age for the overlying tan and orange clayey siltstones found in the vicinity of Red Hill, as well as the Hancock Mammal Quarry (Pratt 1988; Hanson 1996; Bestland et al. 1999). Clarno Formation strata are disconformably overlain by the basal units of the John Day Formation (Peck 1964; Robinson 1975; McClaughry et al. 2009). Rocks of the John Day Formation are interpreted as being derived from a caldera complex distributed across central and eastern Oregon (Ferns et al. 2001; McClaughry and Ferns 2007; McClaughry et al. 2009), with the Wildcat Mountain caldera (41.50 to 39.35 Ma) serving as the likely source of some upper Clarno Formation rocks and the John Day Member A ash-flow tuff (McClaughry et al. 2009). The Member A ash-flow tuff, within the Big Basin Member and at the base of the John Day Formation (Peck 1964; Robinson 1975), has been ⁴⁰Ar/³⁹Ar dated at 39.22 ± 0.03 Ma in the Clarno area (Bestland and Retallack 1994) and U/Pb dated at 39.677 ± 0.014 Ma from Clarno (Mohr et al. 2025). Based on these radiometric dates, the age of the Hancock Mammal Quarry is latest Uintan or Duchesnean NALMA (late Lutetian or Bartonian).

The Clarno Formation is well-known for the incredibly diverse array of plant fossils found there, with the Clarno Nut Beds renowned as one of the best palaeobotanical sites of any age on the planet (Scott 1954; Manchester 1981, 1990, 1994; Scott and Wheeler 1982; Wheeler and Manchester 2002; Dillhoff et al. 2009). In addition to fossil plants, several Clarno sites preserve isolated pockets of vertebrate and invertebrate fossils, which represent the oldest Cenozoic records of animals from the Pacific Northwest. The only reptiles known from the Nut Beds are a few specimens of the planocraniid crocodylian Boverisuchus Kuhn, 1938 and a single peripheral of the tortoise Hadrianus Cope, 1872 (Hanson 1996; Brochu 2013).

The Hancock Mammal Quarry is the only vertebrate quarry deposit published from the John Day Basin (Pratt 1988; Hanson 1996; Fremd 2010). While typically discussed as a single entity, the quarry actually contains a series of strata, including some leaf bearing shales that contain fish and amphibians (Bestland and Retallack 1994; Fremd 2010). The fauna from the quarry was primarily described by Hanson (1996), with most material coming from a few abundant large mammals. The quarry beds include a concentration of disarticulated large mammal skeletons, probably having been deposited in a point bar of a meandering river channel (Pratt 1988). Of fourteen known species, the most common mammals in the quarry are an early rhino, Teletaceras Hanson, 1989 and a large brontothere, Eubrontotherium Mihlbachler, 2007. Other commonly represented mammals in the quarry include a ‘marsh rhino’ (Zaisanamynodon Belyaeva, 1971), tapir (Plesiocolopirus Schoch, 1989) and ‘clawed oreodont’ (Diplobunops Peterson, 1919) (Hanson 1996; Lucas 2006; Emery et al. 2016). A few rare specimens of a ‘running rhino’ (Hyracodon Leidy, 1856), early horses (Epihippus Marsh, 1875 and Haplohippus McGrew, 1953) and several other artiodactyls (Achaenodon Cope, 1873 and Heptacodon Marsh, 1894) are recognised from the quarry (Hanson 1996; Lucas et al. 2004). Other rare mammals from the quarry include predators: a hyaenodontid creodont Hemipsalodon Cope, 1885 (Mellett 1969), a mesonychid Harpagolestes Wortman, 1901 (Robson et al. 2019) and an unidentified nimravid (Hanson 1996). Small mammals are uncommon in the quarry; to date, only a few rodent incisors have been recovered and described by Samuels and Korth (2017). Coincidentally with the Nut Beds mentioned earlier, the only reptiles recovered from the site are a crocodylian and a turtle: the fragments of the alligatoroid described herein and a chelydrid (Hanson 1996).

The Hancock Quarry shares taxa considered characteristic of multiple land mammal ages and has been referred to both the Uintan and Duchesnean North American Land Mammal Ages (Hanson 1996; Lucas 2006; Robinson et al. 2004). Epihippus is considered a Uintan taxon, while Hemipsalodon, Haplohippus and Plesiocolopirus are considered Duchesnean taxa (Robinson et al. 2004) and the agriochoere Diplobunops and mesonychid Harpagolestes are known from both ages (Emery et al. 2016; Robson et al. 2019). Fortunately, the Hancock Quarry has a well-constrained age, based on radiometric dates, between 42.7 and 39.677 Ma (Bestland and Retallack 1994; Mohr et al. 2025), which would place the site within the latest Uintan (Ui3) or early Duchesnean (Lucas et al. 2004; Samuels and Korth 2017). This age range, straddling the Uintan–Duchesnean boundary, might explain the mixture of taxa from multiple land mammal ages. Interestingly, Harpagolestes, Eubrontotherium, Zaisanamynodon, Plesiocolopirus and Teletaceras known from the Hancock Quarry also occur in parts of Asia at the same time (Radinsky 1963; Lucas 1992; Hanson 1996; Lucas 2006; Mihlbachler 2007, 2008).

While known primarily for the vertebrates preserved at the site, there is also a small flora that has been identified from the Hancock Mammal Quarry units (McKee 1970; Bestland and Retallack 1994). All plants from the quarry are angiosperms and the lack of thermophilic species, which dominate the Nut Beds and the palaeosols from this interval may indicate seasonally cooler and drier conditions at the time (Bestland et al. 1999; Dillhoff et al. 2009).

UCMP, University of California Museum of Paleontology, Berkeley, California; YPM VPPU, Yale Peabody Museum Princeton University Vertebrate Paleontology Collection, New Haven, Connecticut, all USA.

UCMP 164865 from the Clarno Formation shares traits with several alligatoroid taxa, but exhibits a unique suite of character states. Discrete morphological data are shared amongst several taxa of alligatorids, alligatorines, caimanines and orientalosuchins and offers some insight into possible taxonomic relationships. The combination of three phylogenetic characters (a short dentary symphysis, a deeply curved dentary and a splenial that participates in the lower jaw symphysis) unites the HMQ alligatoroid with Dongnanosuchus hsui and Alligator olseni (Walter et al. 2025). Further similarity can be made with D. hsui in the deeply-furrowed lingual dental striations. With the alligatorid Ahdeskatanka and the caimanines Paleosuchus and Bottosaurus, it shares labiolingually compressed teeth, anterior and posterior carinae and relatively large and widespread teeth caudal to the fourth.

The basiapical striations on the lingual surface of the teeth of UCMP 164865 may also offer some phylogenetic information. Basiapical striations are common in the teeth of many crocodylians and may serve a purpose towards prey manipulation (MacKenzie et al. 2024). While lateral striations are common on many crocodylians (MacKenzie et al. 2024), deeply furrowed basiapical striations are especially present in several Eocene orientalosuchin taxa: Krabisuchus (Martin and Lauprasert 2010), Orientalosuchus (Massonne et al. 2019) and on the lingual margins of Dongnanosuchus (Shan et al. 2021). It is possible that this condition is synapomorphic in Orientalosuchina.

While UCMP 164865 can be confidently assigned to Alligatoroidea based on preserved characters, more exclusive taxonomic assignment cannot be made at this time. Though the earliest assignable members of Alligator are late Eocene (Chadronian), they possess a mandibular symphysis extending to nine alveoli (Whiting and Hastings 2015) and exhibit anterior dentary morphology inconsistent with that of the HMQ alligatoroid, precluding its assignment to Alligator. Dongnanosuchus hsui is known from the Eocene of eastern Asia (China) (Shan et al. 2021) and all representatives of Orientalosuchina originate from East and Southeast Asia (Martin and Lauprasert 2010; Wang et al. 2016; Li et al. 2019; Massonne et al. 2019; Shan et al. 2021; Wu et al. 2023). There is well-documented and repeated faunal exchange between East Asia and North America via Beringia during the Mesozoic and Cenozoic (e.g. Woodburne and Swisher III (1995); Hutchison (2000); Godefroit et al. (2003); Loewen et al. (2013); Farke et al. (2014); Jiang et al. (2019)) and discovering a representative of Orientalosuchina in the western part of North America would not be unexpected. Another reason for this possibility is the location of the specimen described here, in the Clarno Formation itself. The site contains several floral and faunal taxa with eastern Asian connections (Radinsky 1963; Lucas 1992; Hanson 1996; Lucas 2006; Mihlbachler 2007, 2008; Dillhoff et al. 2009). That the HMQ alligatoroid is indeed a North American orientalosuchin similar to D. hsui therefore is at least possible, if not plausible.

An interesting correlate to the possible taxonomic relationship of the HMQ alligatoroid is its geographic position in North America: namely, its location west of the Rocky Mountains. The Laramide Orogeny was the primary uplift event forming the Rocky Mountains and occurred between the late Cretaceous through early Eocene (English and Johnston 2004) and, thus, could have posed a potential barrier to crocodylian dispersal by the Uintan. Alligatoroids are common in the early Cenozoic east and south of the Rockies (Brochu 1999, 2004, 2010; Whiting and Hastings 2015; Stocker et al. 2021; Cossette and Tarailo 2024), but virtually absent west of the range (the notable exception being UCMP 164865) (Fig. 1).

Many of the mammalian genera from the Hancock Mammal Quarry are also known from Asia (Radinsky 1963; Lucas 1992; Hanson 1996; Lucas 2006; Mihlbachler 2007, 2008). Amongst the 14 genera known from the site, Harpagolestes, Teletaceras, Eubrontotherium, Plesiocolopirus and Zaisanamynodon also occur in parts of Asia in the middle Eocene. The tortoise Hadrianus from the slightly older Nut Beds, is also known from the early Eocene of Asia (Danilov and Averianov 1997). Approximately 40% of genera from the Nut Beds flora of the Clarno Formation are present in the Eocene floras of Europe and Asia, suggesting there was a mid-latitude Northern Hemisphere circumglobal forest during the Eocene that was connected by Bering and North Atlantic land bridges (Wing 1998; Graham 1999; Tiffney and Manchester 2001; Dillhoff et al. 2009). This combination of faunal and floral connections between the Clarno Formation and biotas of Asia support the idea that an orientalosuchin may have dispersed to North America in the Eocene. The geologic and palaeontologic context lends circumstantial plausibility to the hypothesis that this small, enigmatic alligatoroid from the Clarno Formation of Central Oregon (Fig. 5) is indeed an emigrating orientalosuchin or that the clade occupied a wider distribution than the known fossil record suggests.

Figure 5. 

Artistic reconstruction of small alligatoroids preparing to bask in the Eocene of central Oregon, as a distant volcano begins to contribute the volcaniclastic sediments that will preserve organisms at the site. Reconstruction is based on the hypothesis that the HMQ taxon is a member of a basally-branching alligatoroid clade. Image created by Cody Rex-Allen Stevenson (2025).